In skillet saute onions in butter, but do not brown. Slowly add flour followed by half and half, stirring constantly to make a smooth white sauce. Add remaining ingredients, except cheeses, mushrooms, and shrimp. Cook uncovered, on medium heat, stirring constantly until thick. Add cheeses, and cook, stirring constantly, until melted. Add shrimp and mushrooms about 5 minutes before serving. Serve over rice or in pastry shells. May be made ahead, frozen, and reheated slowly on low heat when time to serve.
Cajun Country -- New Iberia, Louisiana